Remove more references to the old plugin system

pkg/analysis_server/test/context_manager_test.dart

Index: pkg/analysis_server/test/context_manager_test.dart
diff --git a/pkg/analysis_server/test/context_manager_test.dart b/pkg/analysis_server/test/context_manager_test.dart
index f1cd504f8a7df150ee6099b85896fd8d4bf565a7..ddabf0eded385787ee5c81959f8b7c280d6011dd 100644
--- a/pkg/analysis_server/test/context_manager_test.dart
+++ b/pkg/analysis_server/test/context_manager_test.dart
@@ -32,7 +32,6 @@ import 'package:linter/src/rules.dart';
 import 'package:linter/src/rules/avoid_as.dart';
 import 'package:path/path.dart' as path;
 import 'package:plugin/manager.dart';
-import 'package:plugin/plugin.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:watcher/watcher.dart';
@@ -1769,10 +1768,8 @@ abstract class ContextManagerTest {
   }
 
   void processRequiredPlugins() {
-    List<Plugin> plugins = <Plugin>[];
-    plugins.addAll(AnalysisEngine.instance.requiredPlugins);
     ExtensionManager manager = new ExtensionManager();
-    manager.processPlugins(plugins);
+    manager.processPlugins(AnalysisEngine.instance.requiredPlugins);
 
     registerLintRules();
   }